1. Field of the Invention
The present invention relates to an induction furnace used for melting metal, such as a crucible type induction furnace or groove type induction furnace in which metal is molten by induction heating, and the resultant molten metal is kept at a desired temperature.
2. Description of the Related Art
A conventional induction furnace, used for example to melt metal, includes a furnace body of refractory material, encircled by an induction coil. If this conventional furnace body is cracked, then the molten metal may leak out to touch the induction coil, thus causing a great accident. In order to detect the occurrence of such an accident, the induction furnace is typically provided with a molten metal leakage detecting unit.
Such molten metal leakage detecting units have been disclosed by Japanese Utility Patent Application (OPI) No's 101792/1988 and 159892/1984, Japanese Patent Application (OPI) No. 182568/1987, and Japanese Utility Patent Application Publication 7278/1983 (the term "OPI" as used herein means an "unexamined published application").
A typical example of the conventional molten metal leakage detecting units will be described with reference to FIG. 4. When its furnace body 1 of a crucible type induction furnace of refractory material is abnormally damaged, or cracked, the molten metal 3 will leak out of the furnace body. The leakage of the molten metal is detected as follows. A first electrode 4 is provided in the bottom of the furnace body 1 in such a manner that it is in contact with the molten metal 3 of the furnace body 1. A second electrode 5 of aluminum or stainless steel foil is located between an induction coil 2 wound on the furnace body 1 and the outer surface of an heat insulating layer covering the outer cylindrical wall of the furnace body 1. Outside the furnace body 1, a predetermined voltage is applied across the first and second electrodes 4 and 5. When the molten metal 3 leaks out of the furnace body 1 through the cracks or the like, and contacts second electrode 5, a current path is created between first and second electrodes 4 and 5 via the molten metal 3. Thus current will flow between the two electrodes, and be detected with an ampere meter 6. In other words, deflection of ampere meter 6 indicates presence of a leak. In response to the detection, an audible alarm signal is produced.
While this invention relates in part to the above-described molten metal leakage detecting technique, it relates more specifically to a furnace building technique which will make the above leak-detection technique unnecessary and obsolete. A furnace body is constructed by sintering castable refractory as follows. First, a coil assembly is formed by covering coil inductors in such a manner that they are insulated from one another and from ground. Granular refractory, or stamp material is put inside the coil assembly thus formed, and is then hardened by pounding to form a furnace bottom. With a metal form set on the furnace bottom, similar stamp material is hardened by pounding to form the side wall of the furnace body. Thereafter, a metal form and ground metal are put in the furnace body. After being sintered at a low temperature with a burner, the ground metal is made molten through induction heating the ground metal with the energized, coil, thereby sintering the stamp material in the furnace. In this case, the metal form is melted together with the ground metal, or a metal form having a high melting point is used, so that it can be removed and used again.
A conventional furnace building method of this type has been disclosed by Japanese Patent Application Publication No. 53190/1981. In the method, boron oxide (B.sub.2 O.sub.2) is used as a sintering agent for silica stamp material so that, unlike a conventional method of using boric acid (H.sub.3 BO.sub.3) the adverse effect of water content, that is a dehydrating reaction during sintering, is eliminated.
In a furnace utilizing the earlier-described molten metal leakage detecting technique, even if the damage to the furnace body is not serious, water vapor present in the refractory material comprising the furnace body (such as natural silica), or the metallic vapor provided when the metal billets to be sintered are low melting point material (such as lead or zinc, zinc-plated iron plates, or zinc-copper alloy), will occasionally leak out of pores in the furnace body during the furnace operation.
As is apparent from the above description, even when the furnace body is not serious, with only slight steam or metallic vapor leaks, and even when the peripheral components of the furnace body, especially the induction coil, are not adversely affected, it appears that a molten metal leak has occurred. An unnecessary furnace repair operation may result. As was described above, the furnace body is formed by hardening granular refractory material through pounding. Accordingly, the furnace body has a number of pores. Hence, even when the furnace body is in normal state, the vapor of low melting point metal (such as lead or zinc) will sometimes pass through the pores, causing the leak detecting means to give erroneous indications.
In the above-described conventional furnace building technique, no moisture is produced by the dehydration of boron. However, with stamp materials such as silica stamp material, magnesia stamp material, alumina stamp material, mullite stamp material and spinel stamp material, moisture is produced because of the water contained therein. If, during sintering, the quantity of steam produced and the quantity of steam discharged become greatly different from each other, then cavities are formed in the refractory material. The resultant structure is low in refractoriness. Thus, the quick temperature increase of the metal form is limited.